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LTspice Simulation Commands Reference

Complete reference for all dot commands (simulation directives) in LTspice.

Table of Contents

  1. Analysis Types
  2. Measurement & Output
  3. Parameter Control
  4. Simulator Configuration
  5. Circuit Structure
  6. State Management
  7. Advanced

Analysis Types

.TRAN — Transient Analysis

Simulates circuit behavior over time when powered up.

.tran <Tstop> [modifiers]
.tran <Tstep> <Tstop> [Tstart [dTmax]] [modifiers]
Parameter Description
Tstep Plotting increment / initial step-size guess (can be 0)
Tstop Duration of simulation (required)
Tstart Start time for saving data (data before this discarded)
dTmax Maximum time step

Modifiers:

Modifier Description
uic Skip DC operating point, use initial conditions
steady Stop when steady state reached
nodiscard Keep data before steady state
startup Solve with sources off, ramp on in first 20u
step Compute step response
convreport Add convergence scores to log

State file options: loadstate[=<file>], savestate[=<file>], savestatetime=<time>

Examples:

.tran 5u
.tran 0 1m startup
.tran 10n 100u 0 10n
.tran 1m steady savestate

.AC — AC Analysis

Small-signal AC analysis linearized about the DC operating point.

.ac <oct|dec|lin> <Nsteps> <StartFreq> <EndFreq>
.ac list <Freq1> [<Freq2> ...]
.ac file=<filename>
Parameter Description
oct Logarithmic, Nsteps per octave
dec Logarithmic, Nsteps per decade
lin Linear, Nsteps total
Nsteps Number of frequency points
StartFreq Starting frequency
EndFreq Ending frequency

Examples:

.ac dec 100 1 1Meg
.ac oct 10 100 100K
.ac lin 1000 1K 10K
.ac list 60 120 1K 10K 100K
.ac file=freq_list.txt

.DC — DC Sweep

Sweeps the DC value of one or more independent sources. Up to 3 nested sweeps.

.dc [oct|dec|lin] <srcnam> <start> <stop> <incr|points>
.dc <srcnam> list <val1> <val2> [<val3> ...]
.dc <srcnam> file=<filename>

Nesting (up to 3 sweeps):

.dc Vds 0 5 0.05 Vgs 0 5 1

Examples:

.dc V1 0 5 0.1
.dc Vds 3.5 0 -0.05 Vgs 0 3.5 0.5
.dc I1 0 2m 0.1m
.dc V1 list 1 2.5 5
.dc dec V1 1 100 10

.OP — DC Operating Point

Finds DC operating point (capacitors open, inductors shorted).

.op

No parameters. Results appear in dialog and status bar. Usually performed automatically as part of other analyses.

Operating point methods (tried in order):

  1. Direct Newton iteration
  2. Adaptive Gmin stepping
  3. Adaptive source stepping
  4. Pseudo transient

Use .options logopinfo to log semiconductor operating point information.

.NOISE — Noise Analysis

Computes noise spectral density (Johnson, shot, flicker sources).

.noise V(<out>[,<ref>]) <src> <oct|dec|lin> <Nsteps> <StartFreq> <EndFreq>
.noise V(<out>[,<ref>]) <src> list <Freq1> [<Freq2> ...]
.noise V(<out>[,<ref>]) <src> file=<filename>
Parameter Description
V(out[,ref]) Output node(s) for noise calculation
src Reference source (input-referred noise)

Output traces:

  • V(onoise) — output-referred noise voltage density
  • V(inoise) — input-referred noise density

Ctrl+click on trace label to integrate noise over bandwidth.

Example:

.noise V(out) Vin dec 100 1 10Meg

.TF — Transfer Function

DC small-signal transfer function analysis.

.TF V(<node>[,<ref>]) <source>
.TF I(<Vsource>) <source>

Examples:

.TF V(out) Vin
.TF V(5,3) Vin
.TF I(Vload) Vin

.FRA — Frequency Response Analysis

Time-domain frequency response analysis for feedback loops (e.g., SMPS stability). Requires an FRA device instance (prefix @) — the sweep range, stimulus amplitude and timing are all set on that device, not on this command. See CIRCUIT-ELEMENTS-REFERENCE.md for its parameters. Optional FRA probe devices (prefix &) add differential measurement points to the same run, and a circuit with multiple independent loops can use one FRA device per loop.

.fra [Tstart=<val>] [dTmax=<val>] [Tstep=<val>] [Tstop=<val>]
+ [uic] [startup] [loadstate[=<file>]] [savestate[=<file>]]

All parameters optional, specified by keyword. FRA automatically stops when all FRA devices complete analysis.

Follow the step-by-step procedure in SMPS Bode Plots (FRA) rather than configuring the analysis from scratch. A valid measurement depends on device settings that have to be established in order — a delay long enough to reach steady state, a stimulus amplitude that does not disturb the operating point, and adequate settling and averaging time at each frequency. Misset, they yield a plausible-looking Bode plot that is simply wrong.

Measurements on FRA data: an .fra run is a transient run — its .raw file reports Transient Analysis with time as the x-axis — so .meas TRAN statements placed in the circuit work directly, alongside the frequency-response results. No separate .tran simulation is needed to get ripple, overshoot, or average figures out of the same run.

Window the measurement, though. With no range given it spans the whole FRA sweep, which is dominated by the stimulus the FRA device injects; use FROM/TO (or TRIG/TARG) to restrict it. The interval before the device’s delay is still unperturbed.

.meas TRAN vout_pp   PP V(out)                 ; whole sweep — includes FRA stimulus
.meas TRAN quiescent PP V(out) FROM 0 TO 100u  ; before delay=100u — stimulus not yet applied

.meas AC does not work on FRA results. Every .meas statement in an FRA circuit is evaluated against the transient data, so a .meas AC directive on an FRA schematic is silently discarded — it produces no result, no warning, and no mention of the measurement name anywhere in the log. There is no way to measure the frequency-domain (Bode) data from a directive in the circuit.

To measure the Bode data, run the measurements against the FRA plot from the waveform window instead: make the FRA plot the active window and use File > Execute .MEAS Script (see .MEASURE). That is the only route to .meas results on the frequency response LTspice extracts from the time-domain run.

See: File > Open Examples > Educational\FRA\

.FOUR — Fourier Analysis

Computes Fourier series components after transient analysis. Output in .log file.

.four <frequency> [Nharmonics] [Nperiods] <trace1> [<trace2> ...]
Parameter Description Default
frequency Fundamental frequency —
Nharmonics Number of harmonics 9
Nperiods Periods to analyze (-1 = all data) 1 (last period)

Example:

.four 1K V(out)
.four 60 15 V(output) I(Vsupply)

Measurement & Output

.MEASURE — User-Defined Measurements

Post-processing command to extract measurements from simulation results.

Single-Point Measurement

.meas [TRAN|AC|DC|NOISE] <name> <FIND|DERIV|PARAM> <expr>
+ [WHEN <condition> | AT=<value>]
+ [TD=<delay>] [RISE|FALL|CROSS=<count>|LAST]

Range Measurement

.meas [TRAN|AC|DC|NOISE] <name> <AVG|MAX|MIN|PP|RMS|INTEG> <expr>
+ [TRIG <expr> [VAL=]<val> [TD=<val>] [RISE|FALL|CROSS=<count>]]
+ [TARG <expr> [VAL=]<val> [TD=<val>] [RISE|FALL|CROSS=<count>]]

Range operations:

Operation Description
AVG Average over range
MAX Maximum value
MIN Minimum value
PP Peak-to-peak
RMS Root mean square
INTEG Integral

Examples:

.meas TRAN Vmax MAX V(out)
.meas TRAN Trise TRIG V(out)=0.5 RISE=1 TARG V(out)=4.5 RISE=1
.meas TRAN Pwr AVG V(out)*I(Vout)
.meas TRAN Vfinal FIND V(out) AT=10u
.meas TRAN Vcross FIND V(out) WHEN V(clk)=2.5 CROSS=3
.meas TRAN delay PARAM Trise*2
.meas AC fc WHEN mag(V(out))=mag(V(out))/sqrt(2) FALL=1
.meas AC BW TRIG mag(V(out))=tmp/sqrt(2) RISE=1 TARG mag(V(out))=tmp/sqrt(2) FALL=LAST
.meas NOISE total_noise INTEG V(onoise)

Worked examples: MEAS-REFERENCE.md has complete netlists paired with the log output they produce, covering AC and noise measurements in depth.

Output: Results in .log file. With .step, results form tables. Data saved to SQLite .db file (see MEASURE-DATABASE-REFERENCE.md).

Note: The output of one .meas statement can be used in other .meas statements (e.g., PARAM Trise*2 references the Trise measurement).

Viewing stepped .MEAS results in LTspice:

  1. After simulation completes, open View > SPICE Output Log
  2. Right-click in the log file
  3. Execute Plot .step’ed .meas data from the context menu

This plots the .MEAS results as waveforms indexed by step parameter value.

Running .MEAS on an existing dataset (no re-simulation): .meas statements are evaluated entirely in post processing, so a script of them can be executed against waveform data already on disk. Make the waveform window the active window, then use File > Execute .MEAS Script. This avoids re-running the simulation just to add or change a measurement. The script file may be an ordinary netlist — everything except the .meas statements is ignored, so the circuit’s own .net/.cir file can be used directly.

Accuracy caveat: because .meas reads the saved waveform data, its accuracy is limited by that data after compression. Disable or loosen compression (.options plotwinsize=0) for more precise .meas output — see Waveform Compression.

.SAVE — Limit Saved Data

Restricts saved output to specified traces (reduces file size).

.save V(out) I(L1) I(R2)
.save V(*) Id(*)
.save V(x23:*)

Supports wildcards * and ?. Use : for hierarchy (e.g., V(x23:node1)).

.WAVE — Output WAV File

Writes simulation data to a .wav audio file.

.wave <filename.wav> <Nbits> <SampleRate> V(out) [V(out2) ...]
Parameter Range
Nbits 1–32
SampleRate 1–4,294,967,295 Hz
Channels 1–65,535

Full-scale range: -1V to +1V (or -1A to +1A).

Example:

.wave C:\output.wav 16 44.1K V(left) V(right)

Parameter Control

.PARAM — User-Defined Parameters

Define constants and expressions for parameterized circuits.

.param <name>=<value>
.param <name>=<expression>
.param <name>="<string>"

Built-in constants:

Name Value
pi 3.14159265358979323846
BOLTZ 1.3806503e-23
ECHARGE 1.602176462e-19
PLANCK 6.62620e-34
KELVIN -273.15
GMIN 1e-12

Available functions: abs, acos, asin, atan, atan2, cos, sin, tan, cosh, sinh, tanh, exp, ln, log10, sqrt, cbrt, pow, pwr, pwrs, int, floor, ceil, round, buf, inv, u, uramp, sgn, if, limit, min, max, hypot, rand, flat, gauss, mc, mod, select, table, xor

Operator precedence (low to high): & | ^ → > < >= <= == != → + - → * / % → **

String parameters: Can parameterize model/subcircuit names.

.param model_name = select(n, "1N4148", "1N4007")

Examples:

.param Rload=10K
.param freq=100K
.param RC_time=Rload*100n
.param pi2=2*pi

.FUNC — User-Defined Functions

Create reusable functions.

.func <name>([args]) {<expression>}

Example:

.func Pythag(x,y) {sqrt(x*x+y*y)}
.func dBV(x) {20*log10(x)}

R1 a b {Pythag(300,400)}   ; = 500 ohms

Uses dynamic scoping: names resolved where function is called.

.STEP — Parameter Sweeps

Repeatedly run analysis while sweeping a parameter. Multiple .step directives nest, multiplying the run count: n two-value sweeps produce 2ⁿ runs.

.step [oct|dec|lin] <item> <start> <end> <incr|points>
.step <item> list <val1> <val2> [<val3> ...]
.step <item> file=<filename>

Item formats:

Item Syntax Notes
Parameter param RLOAD The param keyword is required
Source V1 or I1 Voltage or current source name
Temperature temp  
Model parameter NPN 2N2222(VAF) The model type prefix is required

Rules:

  • At least two steps are required. A spec resolving to one step (list with one value, duplicate-only values, start == end, or a zero increment) is rejected and the simulation does not run at all.
  • oct|dec|lin precedes the item, and cannot combine with list.
  • The third argument depends on the keyword: an increment for bare/lin, but points per decade for dec and per octave for oct.
  • list and file= take plain numbers — suffixes and scientific notation are fine, {} expressions are not. A file= list may be newline- or space-separated.

Examples:

.step param Rload 1K 10K 1K
.step param Rload list 1K 2.2K 4.7K 10K
.step V1 0 5 0.5
.step temp -40 125 5
.step NPN 2N2222(BF) 50 200 50
.step dec param freq 1K 1Meg 10
.step param Rload file=rvalues.txt

.TEMP — Temperature Sweeps

Archaic shorthand for .step temp list ....

.temp <T1> [<T2> ...]

Example:

.temp -55 25 85 125

Simulator Configuration

.OPTIONS — Simulator Options

Control simulator tolerances, integration method, waveform compression, and diagnostic output.

.options <keyword>=<value> [<keyword>=<value> ...]
.options <flag>

Convergence & Accuracy

Option Default Description
abstol 1p Absolute current tolerance
vntol 1u Absolute voltage tolerance
reltol 0.001 Relative error tolerance
chgtol 10f Absolute charge tolerance
trtol 2.0 Transient truncation error factor
gmin 1e-12 Min conductance on PN junctions
method trap Integration: trap or gear

Iteration Limits

Option Default Description
itl1 100 DC iteration limit
itl2 50 DC transfer curve iteration limit
itl4 10 Transient iteration limit per timepoint
gminsteps 25 Gmin stepping iterations (0=disable)
srcsteps 25 Source stepping iterations (0=disable)
ptrantau 0.1 Pseudo-transient time constant (0=disable)

Time Step Control

Option Default Description
maxstep Tstop/1024 Maximum transient step size
solver — Matrix solver: “norm” or “alt”

Waveform Compression

Option Default Description
plotreltol 0.0025 Relative tolerance
plotvntol 10u Absolute voltage tolerance
plotabstol 1n Absolute current tolerance
plotwinsize 300 Points per window (0=disable)

Convergence Aids

Option Default Description
gshunt 0 Conductance to ground from every node
cshunt 0 Capacitance to ground from every node
gfloat 1e-12 Conductance for floating nodes

Temperature

Option Default Description
temp 27 Default simulation temperature (C)
tnom 27 Model parameter measurement temperature (C)

Diagnostics

Option Default Description
numdgt 6 Significant digits (>6 = double precision)
measdgt 12 .MEASURE output digits
list off Expanded netlist in log
logparams off All parameters in log
logopinfo off Semiconductor OP info in log
debugtran off Convergence difficulty scores
topologycheck 1 Check floating nodes, voltage loops

Steady-State Detection

Option Default Description
sstol 0.001 Steady-state relative tolerance
ststdelay 0 Delay before detection starts
ststclocks 10 Clock cycles after steady state

Example:

.options reltol=1e-4 method=gear
.options maxstep=1u gshunt=1e-12
.options numdgt=15 plotwinsize=0

Circuit Structure

.SUBCKT / .ENDS — Subcircuit Definition

.subckt <name> <port1> <port2> ... [params: p1=val1 p2=val2]
  [circuit elements]
.ends [<name>]

Instantiated with X element:

X1 node1 node2 node3 <subckt_name> [param1=val1]

Example:

.subckt divider A B C
.param top=1K bot=1K
R1 A B {top}
R2 B C {bot}
.ends divider

X1 in out 0 divider top=9K bot=1K

.MODEL — SPICE Model Definition

.model <name> <type>[(<param1>=<val1> <param2>=<val2> ...)]

Types: D, NPN, PNP, NJF, PJF, NMOS, PMOS, NMF, PMF, SW, CSW, VDMOS, NIGBT, PIGBT, URC, LTRA

Derived model (inherit and override):

.model SLOW ako:FAST D(tt=10n)

.INCLUDE — Include File

.include <filename>

Inserts entire file contents into netlist. Relative paths resolve from the directory containing the directive.

.LIB — Include Library

.lib <filename>
.lib <filename> <section_name>

Like .include but ignores global-scope circuit elements (only imports models/subcircuits).

Library section format:

.lib <section_name>
  [definitions]
.endl

Search order (relative paths):

  1. Directory of calling netlist
  2. User libraries directory
  3. User search paths
  4. %LOCALAPPDATA%\LTspice\lib\cmp
  5. %LOCALAPPDATA%\LTspice\lib\sub

Encrypted libraries: ltspice.exe -encrypt <filename> (irreversible — backup first!)

.GLOBAL — Global Nodes

.global <node1> [<node2> ...]

Declares nodes as globally accessible (not local to subcircuits). Node 0 is always global. Nodes matching $G_* are automatically global.

.global VDD VCC RESET

.END — End of Netlist

.end

All lines after .end are ignored. Can be omitted. Do not place on schematics (netlister adds it automatically).

State Management

.SAVESTATE — Save Circuit State

Saves complete transient simulation state in proprietary format.

.savestate [<filename>] [time=<value>]
  • Default filename: schematic base name with .state extension
  • Default time: saves final state on completion
  • Multiple .savestate allowed in one simulation

.LOADSTATE — Load Circuit State

Restores previously saved state to resume simulation.

.loadstate [<filename>] [reset]
  • reset: Plot output starting at time zero
  • Circuit must be identical to when state was saved

.SAVEBIAS — Save Operating Point

Saves DC operating point as text file in .nodeset format.

.savebias <filename> [internal] [temp=<val>] [time=<val> [repeat]]
+ [step=<val>] [DC1=<val>] [DC2=<val>] [DC3=<val>]

Superseded by .savestate/.loadstate for transient simulations.

Advanced

.MACHINE — State Machine

Arbitrary state machine definition.

.machine [<tripdt>]
.state <name> <value>
.rule <old_state> <new_state> <condition>
.output (<node>[, <neg_node>]) <expression>
.endmachine
  • First declared state is initial state
  • Rules checked in order; only one fires per timestep
  • * as old state matches any state
  • Condition fires when expression > 0.5
  • state keyword in expressions returns current state value

Example — Divide by 2 with reset:

.machine
.state S0a 0
.state S0b 0
.state S1a 1
.state S1b 1
.rule S0a S0b V(clk) < .5
.rule S0b S1a V(clk) > .5
.rule S1a S1b V(clk) < .5
.rule S1b S0a V(clk) > .5
.rule * S0a V(reset) > .5
.output (out) state
.endmachine

.NET — Network Parameters

Computes S, Y, Z, H parameters during .AC analysis.

.net V(<out>[,<ref>]) <Vin> [Rin=<val>] [Rout=<val>]
.net I(<Rout>) <Vin> [Rin=<val>] [Rout=<val>]

Default termination impedances: 1 Ohm. Terminations don’t affect normal .AC results.

Example:

.net V(out) V1 Rin=50 Rout=50

.BACKANNO — Pin Annotation

.backanno

Automatically included in schematics. Enables cross-probing pin currents by clicking on symbol pins.

See also: CIRCUIT-ELEMENTS-REFERENCE.md for component syntax, TROUBLESHOOTING-GUIDE.md for convergence options

Documentation source: github.com/analogdevicesinc/ltspice-reference